mmc: sd: clean up redundant memset

The clearing of mrq via a memset at the top of the for loop in
mmc_wait_for_app_cmd() is not required as mrq is not used and there is
another clearing of mrq just below.  We remove the first memset since if
the initial tests in the for loop fail the memset is not required.

Signed-off-by: Mark Asselstine <asselsm@gmail.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>